Improve external gateway update handling

Improve external gateway update handling

Once gateway is set, external_gateway_added() was getting called every time
 a router update was received. The check for change in external
gateway compared previously cached copy of gateway port (ri.ex_gw_port) with
 the one passed in through update router (ri.router['gw_port']).
The cached copy was already being modified by code so the two values would
always appear to be different.
Making the change to compare correctly and remove actions not required
for gateway update.

Change-Id: I1a703b327e6c569dfaa8263a222e4bc797e5dbfd
Closes-Bug: 1348737